Apatosaurus (Greek ἀπατέλος or ἀπατέλιος, meaning "deceptive" and σαῦρος meaning "lizard"), also known as Brontosaurus, is a genus of sauropod dinosaurs that lived about 140 million years ago during the Jurassic period. They were some of the largest land animals that ever existed, about 4.5 meters (15 feet) tall at the hips, with a length of up to 21 meters (70 feet) and a mass up to 35 metric tons (40 short tons, with one ton equal to 2,000 lb).

The name Apatosaurus means "deceptive lizard," so-named because the chevron bones (the bones on underside of tail) were like those of Mosasaurus, a large, carnivorous sea-dwelling reptile. Brontosaurus ("thunder lizard") was also a popular name for the animal, the result of Othniel C. Marsh placing two different fossil finds of the same type of dinosaur into two different genera. Stephen Jay Gould attributes this to a too-common rush to publish and name "spectacular dinosaurs," stemming from the famous (and acrimonious) nineteenth century competition for glory between celebrated paleontologists Marsh and Edward Drinker Cope.

The cervical vertebrae and the bones in the legs of Apatosaurus were bigger and heavier than that of Diplodocus although, like Diplodocus, Apatosaurus also had both a long neck and a long tail. Like most sauropods (a suborder or infraorder of dinosaurs), Apatosaurus had only a single large claw on each forelimb. The skull was first identified in 1975, a century after this dinosaur acquired its name.

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Discovery and species

Fossils of Apatosaurus species have been found in the United States at Nine Mile Quarry and Bone Cabin Quarry in Wyoming and at sites in Colorado, Oklahoma, and Utah.

A. ajax is the type species of the genus, and was named by the paleontologistOthniel Charles Marsh in 1877 after Ajax, the hero from Greek mythology. It is the holotype for the genus (a physical example known to be used when the species was formally described) and two partial skeletons have been found, including part of a skull.

A. excelsus (originally labeled Brontosaurus) was named by Marsh in 1879. It is known from six partial skeletons, including part of a skull, which have been found in Oklahoma, Utah, and Wyoming.

A. louisae was named by William Holland, in 1915. It is known from one partial skeleton, which was found in Colorado, in the United States.

Robert T. Bakker made Apatosaurus yahnahpin the type species of a new genus, Eobrontosaurus in 1998, so it is now properly Eobrontosaurus yahnahpin. It was named by Filla, James, and Redman in 1994. One partial skeleton has been found in Wyoming.

Apatosaurus is a member of the Diplodocidae, along with Diplodocus,Barosaurus, and Seismosaurus, although it is not as closely related to the others as they are to each other and hence placed in its own subfamily Apatosaurinae (Taylor and Naish 2005, Harris 2006).

Apatosaurus/Brontosaurus controversy

Apatosaurus' correct head

The name Brontosaurus has also been used for Apatosaurus, both scientifically and popularly, and at one point which name should be used was a source of controversy.

Gould , in his book, Bully for Brontosaurus, states that this controversy "is a direct legacy of the most celebrated feud in the history of vertebrate paleontology," that is, between Othniel Charles Marsh and Edward Drinker Cope. Their competition to discover new fossils became known as the Bone Wars. Gould explains that this vying for glory fell into a pattern of "rush and superficiality born of their intense competition and mutual dislike." In an effort to "bag as many names as possible," they rushed their publications, often with poor illustrations and inadequate descriptions, sometimes described the same creature twice, gave names to fragmentary materials, and sometimes even described species while the skeleton was still largely underground. Thus, the discoverer of Apatosaurus, Marsh, rushed his findings and inadvertently led to a controversy in names.

In 1877, O. C. Marsh first published notes on his discovery of Apatosaurus ajax, naming and describing it in two paragraphs without illustration. (Gould 1991)

Marsh followed this in 1879 with a description of another, more complete, dinosaur specimen. He speculated that the latter specimen represented a new genus and named it Brontosaurus excelsus. He considered it to be related to Apatosaurus, but the earlier description was so lacking that it is not surprising he did not link the two specimens more closely (Gould 1991). He estimated the length to be seventy to eighty feet, versus the fifty feet of Apatosaurus. Because of the skeleton's completeness, Brontosaurus "soon became everyone's typical sauropod, indeed the canonical herbivorous dinosaur of popular consciousness" (Gould 1991).

In 1903, it was surmised by Elmer Riggs of the Field Museum in Chicago that Brontosaurus excelsus was in fact an adult Apatosaurus. The name Apatosaurus, having been published first, was deemed to have priority as the official name. Brontosaurus was relegated to being a synonym.

In the 1970s, it also was ascertained that the traditional "Brontosaurus" image known to all was, in fact, an Apatosaurus excelsus with a Camarasaurus head incorrectly placed on its body (McIntosh and Berman 1975). Marsh had found no skull associated with either Apatosaurus or Brontosaurus, so he mounted the skeletons with the head of this other sauropod genus (Gould 1991).

Paleobiology

Early on, it was believed that Apatosaurus was too massive to support its own weight on dry land, so it was theorized that the sauropod must have lived partly submerged in water, perhaps in a swamp. Recent findings do not support this. In fact, like its relative Diplodocus,Apatosaurus was a grazing animal with a very long neck and a long tail that served as a counterweight. Fossilized footprints indicate that it probably lived in herds. To aid in processing food, Apatosaurus may have swallowed gizzard stones (gastroliths) in the same way that many birds do today, as its jaws lacked molars with which to chew tough plant fibers.

Neck

Apatosaurus is believed to have browsed the tops of trees, on riverbanks. Scientists believe that these sauropods could not raise their necks to an angle of 90 degrees, as doing so would slow blood flow to the brain excessively; blood starting at the body proper would take two or more minutes to reach the brain. Furthermore, studies of the structure of the neck vertebrae have revealed that the neck was not as flexible as previously thought.

Physiology

With such a large body mass, combined with a long neck, physiologists encounter problems determining how these animals managed to breathe.

Beginning with the assumption that Apatosaurus, like crocodilians, did not have a diaphragm, the dead-space volume (the amount of unused air remaining in the mouth, trachea, and air tubes after each breath) has been estimated at about 184 liters for a 30 ton specimen.

Its tidal volume (the amount of air moved in or out during a single breath) has been calculated based on the following respiratory systems:

904 liters if avian

225 liters if mammalian

19 liters if reptilian

On this basis, its respiratory system could not have been reptilian, as its tidal volume would not have been able to replace its dead-space volume. Likewise, the mammalian system would only provide a fraction of new air on each breath. Therefore, it is assumed to have had either a system unknown in the modern world or one like birds, i.e. multiple air sacs and a flow-through lung.

Furthermore, an avian system would only need a lung volume of about 600 liters compared to a mammalian requirement of 2,950 liters, which would exceed the available space. The overall thoracic volume of Apatosaurus has been estimated at 1,700 liters allowing for a 500-liter, four-chambered heart (like birds, not three-chambered like reptiles) and a 900-liter lung capacity. That would allow about 300 liters for the necessary tissue.

Assuming Apatosaurus had an avian respiratory system and a reptilian resting-metabolism (it certainly could not fly), it would need to consume only about 262 liters (69 gallons) of water per day.

It is not known how Apatosaurs ate enough food to satisfy their enormous bodies. It is likely that they ate constantly, pausing only to cool off, drink, or to remove parasites. It is surmised that they slept standing upright. They likely relied on their enormous size and herd behavior to deter predators.

Tail

The tail is believed to have been held above the ground during normal locomotion.

An interesting speculation was reported by Discover Magazine in 1997 about "whipcracking" millions of years ago. Nathan Myhrvold, a computer scientist, carried out a computer simulation of the tail of Apatosaurus, a very long, tapering tail resembling a whip, and concluded that sauropods were capable of producing a crack of over 200 decibels, comparable to the sound of a cannon (Zimmer 1997).

McIntosh, J. S., and D. S. Berman. 1975. Description of the palate and lower jaw of the sauropod dinosaur Diplodocus (Reptilia: Saurischia) with remarks on the nature of the skull of Apatosaurus. Journal of Paleontology 49(1): 187-199.

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